1. Field of the Invention
The present invention relates to materials and methods for selectively recovering metal ions, such as copper, from aqueous streams. More specifically, the invention relates to producing high yields of high purity solid metal-ion-capturing material, by covalently binding a chelating moiety to a solid-phase material. The invention is particularly beneficial for recovering copper from aqueous solutions in the copper mining and copper plating industries, because the solid-phase material is selective to copper in the presence of iron and other metals, and results in energy and material savings compared to conventional copper recovery methods.
2. Related Art
Copper chelating moieties have been used in the past for removing copper from aqueous streams in the copper mining industry. Liquid phase chelates in aqueous-immiscible organic solvents are employed in liquid-liquid phase extraction processes to remove copper from the mining product or waste streams. After the aqueous stream is separated from the solvent stream, the solvent containing the chelated copper is then back-extracted with strong acid in order to remove the copper, and the copper is typically recovered by electro-winning. This solvent extraction/electro-winning (SX/EW) process requires large amounts of flammable and toxic solvent, and large amounts of time and energy. This SX/EW process also requires periodic addition of chelating agent due to losses in the process. One chelating agent used in the conventional SX/EW process is a liquid-phase salicylaldoxime copper capturing moiety.
Other methods of metal ion recovery using salicylaldoxime has been studied and described in the literature. Sarkar et al., in xe2x80x9cSorption recovery of metal ions using silica gel modified with salicylaldoxime,xe2x80x9d Talanta, 42 (1996) pp.1857-1863, describes sorbent materials for metal ions, wherein the sorbent material consists of salicylaldoxime physically adsorbed onto solid phase materials. The Sarkar et al. sorbent synthesis is reported as: 1) refluxing silica gel with 6 M HCl for about 3 hours to remove any contaminating metals such as iron; 2) washing the silica gel with deionized water and drying it under reduced pressure at 150xc2x0 C.; 3) refluxing the dried silica gel with salicylaldoxime in ethanol (10% w/w) at 70-80xc2x0 C. for four hours; and 4) filtering and drying under vacuum to obtain the solid sorbent. Sarkar, et al. reports that IR spectrum of the resulting sorbent shows IR peaks coinciding with peaks of salicylaldoxime itself, suggesting that the sorbent synthesis techniques resulted in the salicylaldoxime being retained within the sorbent as such, without any structural change. Consistent with this finding was that ethanol easily removed all the salicylaldoxime from the sorbent when ethanol was passed over the sorbent. These findings were consistent with the salicylaldoxime being physically adsorbed in the Sarkar et al sorbent, rather than being chemically/covalently bonded to the solid material.
In Hammen, U.S. Pat. No. 5,240,602, chromatographic materials have been described that include a silica-based solid support, an affinity ligand, and a polymeric non-ionic, hydrophilic spacer with a coupling group connecting the affinity ligand to the support. These affinity materials use preferential dissolution to create a chromatographic effect, in which the materials of interest are preferentially, but only temporarily, attracted by the affinity materials. These affinity materials do not chelate materials of interest, and do not capture the materials of interest in any way that requires regeneration of the affinity materials. Therefore, the affinity materials do not provide a way to remove metal ions from a stream. Hammen uses polymers as a spacer between the silica-based solid support and the affinity ligand, including PEG, PVA, PPG, polyethylene dithiol, and polymers of glycine, serine, or threonine. The Hammen polymer is attached to the solid support by a substitution mechanism. Hammen discloses nitrogen-based linkages between the affinity ligand and the solid support, which linkages may be appropriate for a chromatographic use but which would be low in selectivity and prone to degradation in other uses.
What is still needed is a solid-phase metal ion capturing material that is durable, highly selective to the desired metals, and acid/base stable, so that it is appropriate for large-scale metal recovery, regeneration, and re-use. Economical copper-recovery materials and methods are still needed that do not require hazardous and toxic flammable solvents, complex and costly process plant equipment, and high-energy-consumption process steps. What are needed are such materials and methods that lower the cost per pound of recovered copper metal in copper mining and plating industries, and that do so with lowered environmental impact compared to conventional technologies.
The present invention comprises a solid-phase metal ion recovery material including a metal ion capturing agent chemically bound to a solid support, and methods for making the recovery material. The invention further comprises methods of recovering metal ions from aqueous streams, such as mining or metal plating product and waste streams, by direct contact between the aqueous stream and the solid-phase recovery material. The invented materials and methods may eliminate the use of liquid-liquid extraction processes and large volumes of organic solvents conventionally used in metal ion recovery in the mining and plating industries. Metal ion recovery materials according to the invention preferably comprise chelating heads covalently bound to ceramics or ceramic precursors via non-polymeric tethers. The chelating head may be any known chelating ligand, for capturing copper, zinc, uranium, plutonium, or other materials. The tether is an organic compound covalently attached to the chelating head and functionalized by a linker that attaches to an olefin portion of the organic compound. The linker is chosen to be appropriate for the selected solid support, for example, a silica linker for use with silica solids, a titania linker for use with titania solids, an alumina linker for use with alumina solids, etc. Several preferred recovery materials are highly selective for copper (II) in the presence of iron (III) and many other materials that are typically present in the digested earth materials of a conventional copper mining operation. Also, several preferred recovery materials are highly selective for copper in the presence of the materials that are present in a commercial metal plating stream.
The invented metal ion recovery material includes an organic metal-capturing compound, hereafter also called xe2x80x9ccapturing compound,xe2x80x9d or xe2x80x9ccapturing agent,xe2x80x9d that is covalently linked to a solid support. Covalent linkage may include, for example, covalent binding directly to the surface of a silica-based solid. Also, covalent linkage may include condensation of silica-based ceramic precursors into a solid, some to all of which precursors are associated with capturing agent prior to the condensation and/or some to all of which precursors become associated with capturing agent during the condensation.
The metal-capturing compound comprises a chelating moiety or chelating xe2x80x9cheadxe2x80x9d covalently bound to a xe2x80x9ctailxe2x80x9d or xe2x80x9ctetherxe2x80x9d that covalently binds the chelating head to the surface of the solid support, for example, via a silica linker if the solid support is a silica-based material. Thus, the entire linkage between the chelating head and the solid support is chemical, rather than physical, and so is very durable. In addition, the preferred synthesis routes result in the invented metal ion recovery material being acid/base stable.
The chelating head may be any of a variety of chelating heads, which captures and chelates with the material of interest, preferably a metal ion. xe2x80x9cCapturingxe2x80x9d herein means irreversible binding under the existing conditions (pH, temperature, etc.) of the feed stream. Release of the captured metal ions may be effected by a second step wherein the new conditions are significantly altered in some fashion as to cause metal ion release, for example, removal of the solid recovery material from the feed stream followed by acid stripping. xe2x80x9cChelating,xe2x80x9d as known by those of skill in the art, means xe2x80x9cmultiple ionic bonds to a single metal center.xe2x80x9d A preferred, but not the only, chelating head is an oxime molecule, for example, a salicylaldoxime-type molecule, 2-hydroxy benzophenone oximes, and/or 2-hydroxy acetophenone oximes. Or, other chelating moieties such as substituted 8-hydroxy quinolines may be used. By xe2x80x9coxime,xe2x80x9d the inventors mean a molecule comprising the group xe2x80x9cCH(:NOH). By xe2x80x9csalicylaldoxime-typexe2x80x9d molecule, the inventors mean molecules including xe2x80x9cC6H4(OH)CH(:NOH),xe2x80x9d including substituted salicylaldoximes, in which one or more of the cyclic hydrogens may be substituted with other molecules such as OH, F, NH2, NR1R2, SH, or SR, wherein R, R1, and R2 in this list may be alkyl or aryl, for example.
The xe2x80x9ctetherxe2x80x9d comprises an organic portion that includes a carbon-carbon double bond prior to its covalent bonding with the solid support. Preferably, the tether ends in an alpha olefin, and, optionally, may contain other unsaturations. The inventors envision that pure alkyl tails will be desirable in some cases, for example, where a hydrophobic capturing agent is desired. The preferred tether is a single, straight chain ether moiety, for example, comprising an ethylene oxy unit. The inventors believe tether compounds from 1 carbon to about 25 carbons will be most effective, with chains of about 3-25 preferred because the chelating heads attached to such chains will be distanced from the silica far enough to be reached by the metal ions.
To link the xe2x80x9chead and tetherxe2x80x9d combination to a solid, the combination must have a linker at the end of the tether, which renders the entire molecule as a xe2x80x9cfunctionalized capturing agentxe2x80x9d (FCA), that is, head plus functionalized tail. For example, a silica linker may be used, such as Rxe2x80x23Si wherein Rxe2x80x2 is Cl or alkoxy. The silica linker attacks the alpha olefin of the tether, to bind the silicon center to the tether via hydrosililation, forming the FCA. Once the FCA is formed, it may be covalently bound to a solid in two approaches for producing an FCA-functionalized solid.
The first approach for producing an FCA-functionalized solid is hydrolysis in the presence of a pre-formed solid, that is, an existing solid that is provided or formed in processes separate from any process involving the FCA, for example, a ceramic condensed previously to any introduction of FCA. This approach results in a modified ceramic solid, that is, a (pre-formed) solid functionalized by the capturing agent extending out from the solid to distance the chelating head from the solid.
The second approach of producing an FCA-functionalized solid is hydrolysis in the presence of no solid, but, instead, in the presence of solid precursors such as tetraethylorthosilicate (TEOS). This approach forms modified ceramic precursors that condense to form the modified solid. In this second approach, solids are formed and FCA is attached in the same process.
Thus, in the first of the methods described above, the FCA is condensed onto the surface of an already-existing, already-formed silica-based solid support, and the solid may be in various forms. In one embodiment of such a method, the final step of synthesis is done in a slurry containing the silica particles, so that the capturing agent condenses onto silica particles to form a granular recovery material. In another embodiment of such a method, the final step is done by condensing the capturing agent onto existing glass or ceramic objects other than granules, resulting in a surface-modified structure. These silica-particle-based or glass/ceramic-based capturing materials may be used for many applications, for example, the copper recovery process discussed herein, or other selective removal applications.
In the second of the methods described, silica-based ceramic precursors are condensed in situ with FCA, that is, the pre-formed FCA are present and covalently binding with the precursors during the condensation process. In such a method, the FCA may be mixed with a single ceramic precursor such as tetraethylorthosilicate (TEOS), or other precursors, or mixtures of precursors. The mixture of capturing agents and ceramic precursors may be hydrolyzed producing water and ethanol, thereby performing a xe2x80x9csol-gelxe2x80x9d type of process. The material resulting from this sol-gel process may be referred to as a xe2x80x9cxerogelxe2x80x9d and is a silica matrix containing the covalently-bound metal ion chelating molecule (xe2x80x9cheadxe2x80x9d) distanced from the solid by the tether. Included the sol-gel process are in-situ reactions in which FCA covalently binds to a ceramic precursor, followed by condensation of a plurality of precursors into a solid, and also in-situ reactions in which ceramic precursors condense together followed by covalent binding of FCA to the condensed solid.
By careful control and customizing of the sol-gel condensation process, for example, by varying the ratio of ceramic precursor to capturing agent, the size and physical properties of the resulting solid particles can be controlled with a high degree of specificity, allowing custom-sized and custom-made modified solids. The inventors believe the FCA will direct the self-assembly/condensation process so that the chelating head remains accessible to the bulk solvent. The condensation process is believed to yield a high capacity metal ion recovery material due to the chelating heads in the final recovery material being substantially exposed. This creates a high surface area material with high chelating site coverage of the ceramic, for an active and high capacity metal recovery material. These condensed ceramic recovery materials may be used for many applications, for example, the copper recovery process discussed herein, or other selective removal applications.
The preferred methods of manufacture include several features that stream-line the synthesis and that produce highly selective and durable solid materials. Protection and de-protection of the chelating head may be necessary, and may include, for example, manipulation of the dehydration equilibrium between an oxime and its corresponding benzisoxazole. Acidic hydration may be used to de-protect the head to the oxime form. This hydration preferably takes place during use of the recovery material, in the acidic environment of the metal ion aqueous feedstream. The process environment is sufficient to de-protect the isoxazole by driving the hydrodynamic equilibrium process toward ring-opening. This strategy protects the oxime site without additional synthetic transformations, such as typical protection/de-protection of reactive intermediates. The role of this hydration/dehydration equilibrium may be seen in various of the example synthesis schemes below.
Another feature of the invention is that the preferred synthesis steps do not include nitrogen-containing linking groups, which are known to chelate undesired metal cations, such as iron (III), and which lead to lowered selectivity of recovery of the desired metals, that is, typically, lowered copper/iron selectivity. The preferred covalently bonded xe2x80x9ctetherxe2x80x9d between the chelating oxime head group to the silica particle solid support provides a more durable material than chelates that are ionically coordinated or simply physisorbed to a surface of a support. Also, the covalently bonded tether system of the invention offers advantages in cost, durability and efficacy over polymer supports, such as polyamines, that have been suggested. Further, the preferred synthesis routes lead to recovery materials that are stable to any solution with a pH less about 12-13, the pH range above which the silica support would dissolve. The preferred pH operating range is xe2x89xa68 pH, at which pH range copper chelation is strong.
Preferred synthesis routes for creating the functionalized capturing agents and the solid metal recovery materials resulting therefrom are illustrated in three classes: Rearrangement, Ether Addition, and Mitsunobu coupling, with etheric linkage methods, via Mitsunobu coupling or other methods, being the especially-preferred methods. Numerous methods for forming an ether-linkage, such as CDD coupling, etc., are described in the chemical literature. See, for example, March, J., xe2x80x9cAdvanced Organic synthesis, 4th Edition,xe2x80x9d J. Wiley/Interscience, N.Y., N.Y., 1994, Part 2.
Some of the methods within the Rearrangement, Ether Addition and Mitsunobu coupling categories are summarized as follows:
1) Rearrange a benzaldehyde molecule substituted with an ether tail having an alpha olefin end, and react the resulting compound in basic environment to form an allyl-hydroxy-benzaldoxime;
xe2x80x83protect oxime head by dehydration;
xe2x80x83add silica-linker to alpha-olefin end of allyl tail;
xe2x80x83covalently attach silica- to silica-based solid; and
xe2x80x83de-protect chelating head by hydration in acidic environment.
2) Various Ether Addition Routes:
a) Synthesize salicylaldoxime from benzaldehyde;
xe2x80x83protect oxime head by de-hydration to an isoxazole;
xe2x80x83add ether tail by substitution at OH group;
xe2x80x83add silica-linker to alpha-olefin end of ether tail;
xe2x80x83covalent attachment of silica-linker to silica-based solid;
xe2x80x83de-protect chelating head by hydration in an acidic environment.
b) Add ether tail to benzaldehyde via tosylate mechanism;
xe2x80x83form benzaldoxime head;
xe2x80x83protect oxime head by substituting with xe2x80x94Si Rxe2x80x33, wherein Rxe2x80x3 may be alkyl, t-butyl,
xe2x80x83perfluoroalkyl, or phenyl;
xe2x80x83add silica-linker to alpha-olefin end of ether tail;
xe2x80x83condense with ceramic precursors, de-protecting chelating head.
c) Add ether tail to benzaldehyde via tosylate mechanism;
xe2x80x83form benzisoxazole head;
xe2x80x83add silica-linker to alpha-olefin end of ether tail;
xe2x80x83condense with ceramic precursors;
xe2x80x83de-protect chelating head by hydration in an acidic environment.
3) a) Add ether tail with alpha-olefin end to benzaldehyde via Mitsunobu ether
xe2x80x83addition (using diethyl azodicarboxylate and tri-phenyl phosphorous);
xe2x80x83form oxime head;
xe2x80x83protect oxime head by substituting with xe2x80x94SiRxe2x80x33;
xe2x80x83add silica-linker to alpha-olefin end of ether tail;
xe2x80x83condense with ceramic precursors, de-protecting chelating head.
b) Double carbon-carbon bond-containing ether tail addition to a benzisoxazole precursor, such as salicyl aldehyde, followed by isoxazole head formation; or
c) Double carbon-carbon bond-containing ether tail to an isoxazole head;
xe2x80x83via Mitsunobu ether addition, followed by:
xe2x80x83silica-linker addition to alpha-olefin end of tail;
xe2x80x83condensation with ceramic precursors; and
xe2x80x83hydration to de-protect chelating head.
The invented materials and methods selectively recover metal ions without the need for organic solvents, without the need for chelating agent makeup, and without the large energy consumption and process complexity of prior art materials and methods. In copper recovery, for example, the copper-containing waste or product stream is applied directly to the invented solid recovery material, thus, bypassing the prior art liquid-liquid extraction step. The copper is then removed from the solid recovery material, to obtain the copper and to regenerate the solid recovery material, preferably by means of an acid strip step or xe2x80x9cacid wash,xe2x80x9d which may be similar to that done in conventional copper recovery processes that do not use solid recovery materials. Because the copper chelating agent is covalently bound to the silica solid, there is little, if any, loss of copper extraction capability over time. By reducing or eliminating organic solvents and chelating agent loss, copper recovery may be accomplished at a lower cost per pound of recovered copper metal. Also, the invented heterogeneous process, which contacts the metal-ion containing aqueous stream with a solid recovery material, reduces adverse environmental impact because of the reduced use and consumption of chemicals and reduced energy consumption.